The manufacture of polyamide (e.g., nylon-6, nylon 6,6 and the like) involves contacting the polyamide at various process steps with water. For example, following polymerization, a polyamide melt is transformed into chips, flakes, pellets or the like by extruding the polyamide melt into strands which are quenched using process water. The cooled strands are thereafter subjected to chipping, flaking or pelletizing operations using suitable known machinery.
Polyamide oligomers will, however, be transferred from the extruded strands of polyamide melt and into the process water during the quenching operation. The process water used to quench the extruded strands of polyimide therefore will contain significant levels of suspended polyamide oligomers. Over time, these polyamide oligomers will coat process equipment (e.g., pipes, pumps, heat-exchangers and the like) forming a tenacious cake thereon which needs to periodically be removed by cleaning so that the process equipment can function within its design parameters. For example, over time, the cooling water equipment may become so coated with oligomers that insufficient cooling water is capable of being supplied to the process thereby necessitating reduced production levels and/or process shut-down. Any loss or cessation of production results in economic losses which, of course, should be avoided.
In an attempt to minimize such equipment downtime, it has been conventional practice to falter process water containing suspended polyamide oligomers. However, conventional filter elements become blocked quickly by the polyamide oligomer filtrate requiring vigorous back-flushing which again disadvantageously entrains the oligomers in the process water. Conventional depth filters are disadvantageous since they cannot be back-flushed, while standard bed filters (i.e., sand filters) and commonly used filter aids (i.e., diatomaceous earth) are incompatible with the polyamide polymerization processes due to silicate contamination.
Therefore, what has been needed in this art are improvements to filtration apparatus and processes whereby polyamide oligomers may be removed efficiently from process waters containing the same so as to minimize equipment down time. It is toward providing such improvements that the present invention is directed.
Broadly, the present invention involves removing polyamide oligomers from aqueous streams by encapsulating particulate polyamide with the polyamide oligomers so as to form a mass in which the polyamide particles are encapsulated in a matrix of the polyamide oligomers which are thereby removed from the aqueous stream. This mass of polyamide oligomers and encapsulated polyamide polyamide particulates thereby may be easily recycled, e.g., by subjecting the entire mass to depolymerization.
According to a particularly preferred embodiment of the present invention, a filter element surface is first coated with particulate polyamide which serves as a filter aid to trap polyamide oligomers suspended in water. In an especially preferred embodiment of the present invention, the particulate polyamide is coated onto a surface of a bag filter so that polyamide oligomers flowing through the coated bag filter surface will adhere to the polyamide particles forming an oligomer cake thereon which encapsulates the polyamide particles.
The oligomer cake and polyamide particles encapsulated thereby may be easily broken into relatively large-sized solid aggregate particles which are thereby easily recovered simply by momentarily discontinuing (or sometimes momentarily reversing) the differential pressure across the filter surface. Since the polyamide particles which are encapsulated by the oligomer cake are chemically compatible with one smother, the entire particulate aggregate may be recycled to its original state, for example by depolymerization processing so as to recover monomers from which the polyamide is formed (e.g., caprolactam).
The process and apparatus of this invention can be automated so as to allow for periodic back-flushing of the oligomer cake and encapsulated polyamide particles, recovery of the particulate aggregate and re-coating of the filter surface with fresh polyamide particles. In such a manner, therefore, the filter employed in the process and apparatus of this invention can be maintained on-stream for substantially prolonged periods of time without a noticeable decrease in its filtration efficiency. As a direct result, the full capacity of the cooling water can be realized in addition to minimizing equipment downtime.
These and other aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the presently preferred exemplary embodiments thereof.